Vibration-induced fatigue damage and strain induced from other sources is a problem in various structures, including marine risers, which are used in offshore drilling, production, insertion and export. Marine risers span the distance between surface platforms and the seabed and are typically found in two general types: top tensioned risers and catenary risers. See, e.g., U.S. Pat. No. 7,328,741 (incorporated herein by reference for all purposes). However, measurement or estimation of riser fatigue has been difficult or impossible, due to the nature of the risers and the environment in which they are used.
As mentioned in the '741 patent, previous methods involved monitoring of ball/flex joint angle values or other systems that provided a limited set of measurements, mainly of the lower flex joint that do not allow for “real-time” management of the entire riser system. The method outlined in the '741 patent utilizes data from an upper and lower module connected to the upper and lower portions of the riser, respectively, providing dynamic motion and orientation data of the two ends of the riser. There are only general statements on how the data from the two extreme ends of the riser can used to estimate stress at desired locations along the riser: the dynamic motions and orientations from the upper and lower ends of the riser are compared to a “table of models” or “database of vibration signatures” to select the best matching model or signature; then, stresses are determined at a “plurality of riser sections.”
Attempting to determine the dynamic motions and stresses along a riser using only data from two endpoints is highly prone to error. Also, a very large number of predetermined models have to be generated by parameterizing all possible combinations of wind speed/direction, wave height/period/heading, current speed/heading/profile, top tension, mud weight, vessel draft, vessel heading, etc. In addition results from predetermined models are prone to error for complex vibration phenomena such as vortex induced vibration (VIV). Predictive VIV analysis software is currently unable to accurately predict stress and fatigue due to inline vibration, higher harmonics and traveling wave behavior. Therefore, such a method is prohibitive and likely to be inaccurate when applied to riser VIV.
U.S. Pat. No. 7,080,689 (“the '689 Patent”) (incorporated herein by reference for all purposes) discloses a complex system that relies on the presence of multiple sensors along the length of the riser; however, there is no provision for determining fatigue in locations at which there are no sensors. The method outlined in '689 patent requires many additional sources of data, such as: environmental data (wind, waves and current), lower marine riser package (LMRP) position, vessel position using a differential global positioning system (DGPS), and quasistatic position of the riser using acoustic beacons. This data is used in conjunction with the riser dynamic motion data, obtained from accelerometers and inclinometers at several points on the riser, to determine stresses. The numerous additional required measurements make the system prohibitive to procure, install and maintain. In addition, little is said on how stresses are obtained from the data. There is no mention of whether the stresses are computed along the entire riser length or around the circumference, nor whether stresses are only provided at sensor locations. Rather, it is curtly stated that data are “compared with results obtained by the dedicated software DeepDRiser (IFP/Principia™), or other similar software.”
Software such as DeepDRiser and DeepVIV are intended for predictive analysis and not intended for fatigue monitoring. They do not take in riser motion measurements from measured vibration data as an input; instead they take in current profiles and rely on empirical relations to estimate riser stress and fatigue. Such software is limited by the assumptions that are inherent in it. For example, it is well known that most predictive VIV software analysis does not include the effect of the third and fifth harmonics of each excited frequency. In addition much of the software does not model in-line vibration and does not model traveling wave behavior well. Furthermore, the empirical data is typically not obtained from flexible risers; rather, it is obtained from rigid cylinders.
The solution suggested by the '741 patent, however, which does not rely on sensors along the length of the riser, is insufficient to address riser fatigue; it relies on the use of predetermined models, or vibration signatures, with only a few measurement locations; and that reliance results in inaccurate estimations of fatigue in the risers. The '689 patent requires many superfluous measurements and does not include software to reconstruct the stress and fatigue along the entire riser from measured motions at several locations along the riser. Instead, it makes reference to predictive software. As such, both the '741 patent and the '689 patent rely on correlating measurements to predictive analysis and are inadequate.
Since predictive analysis is limited as discussed previously, it has been found much more accurate to reconstruct the stress and fatigue along the entire riser directly from the motion of several measurements along the riser using “reconstructive software.” The inventors are aware of previous attempts at reconstructive software using multiple sensors along a riser to predict fatigue damage. (See, e.g., Shi, C., Manuel, L. and Tognarelli, M. A., 2010, Alternative Empirical Procedures for Fatigue Damage Rate Estimation of Instrumented Risers Undergoing Vortex-Induced Vibration, Proceedings of the 29th OMAE conference, Shanghai, China, OMAE2010-20992 and Kaasen, K. E. and Lie, H., 2003, Analysis of Vortex Induced Vibration of Marine Risers, Modeling Identification and Control Vol. 24(2), pp. 71-85). However, accuracy of previous implementations of such methods decline as the sensor density (number of sensors per unit riser length) decreases, especially when the structure vibrates in high-order modes and exhibits traveling wave behavior.
There is a need, therefore, for method of reconstructing stress along structures that have limited sensors and undergo high-order modes and traveling waves. Various examples of the present invention are useful in, for example, (1) analysis of Vortex Induced Vibration (VIV) of marine risers, where dominant modes correspond to excited modes, (2) civil structures, to estimate loads or stresses in critical structural members or interfaces between members, and (3) automotive and aerospace vehicles, to estimate loads or stresses in critical components or interfaces.
Nothing in this document should be interpreted as a representation that a prior art search has been performed or that there are not other references that an examiner may find to be more relevant to the claims in this document. The above are merely cited as examples by way of background and are not intended to be highlighted as the most relevant references.